Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream electronically so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well suited for application of characters onto irregularly shaped surfaces, including, for example, the bottom of beverage containers.
In general, an ink jet composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wetability of the substrate. Further, the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging, and must permit rapid cleanup of the machine components with minimum effort.
However, despite the many advantages of ink jet printing, it has not, to date, been commercially used to print images on ceramics, glass or other substrates that are subjected high temperatures after being so printed. Several patents and publications have, however, made reference to ink jet compositions for such use. U.S. Pat. No. 5,273,575, for example, relates to ink jet formulations for printing onto ceramics, wherein the formulation comprises a metallic salt that is colorless, in combination with a coloring agent. After printing on the substrate and subsequent heat-curing, the metallic salt is converted to a colored oxide. The salts that are used are soluble in the carrier that is employed.
In accordance with PCT publication PCT/GB92/00342, ink jet formulations are disclosed for printing on substrates such as ceramics and glass that are to be heated after printing. The ink compositions of that disclosure comprise pigment dispersions wherein the median size of the pigment particles is from 0.2 to 2 microns, with less than 20 percent of the particles being less than 0.2 microns in size. A significant problem with such formulations, however, is the instability of the formulations, due to settling upon storage. After settling, redispersion of such pigment particles is very difficult, if not impossible.